The natural product pool, which contains many glycosylated secondary metabolites, is the source of over half the world's drug leads. Carbohydrate appendages often play a key role in drug-target interactions. Therefore, alteration of glycosylation patterns on secondary metabolites is a potential strategy for the generation of novel therapeutics.
Carbohydrates mediate many essential biological processes. For example, the saccharide-containing macromolecules that decorate cell surfaces are vital to a variety of cellular functions including cell-cell recognition, apoptosis, differentiation, and tumor metastasis. In a similar fashion, glycosylated natural products contain sugar attachments essential for their activity and continue to serve as reliable platforms for the development of many of existing front-line drugs (Clardy, J. et al., Nature (2004) 432, 829-837; Thorson, J. S. et al., Curr. Org. Chem. (2001) 5, 139-150). While the diverse chemical space accessible by carbohydrates contributes to a remarkably vast array of biological function (Dobson, C. M. Nature (2004) 432, 824-865), a precise understanding of the relationship between sugars and biological activity remains limited by the availability of convenient and effective glycosylation tools (Langenhan, J. M. et al., Curr. Org. Synth. (2005) 2, 59-8).
Digitoxin (1) is a glycosylated natural product with numerous actions and therapeutic uses. In addition to its well-known cardiac activity, which is mediated by inhibition of the plasma membrane Na+/K+-ATPase (Paula, S. et al., Biochemistry (2005) 44, 498-510), digitoxin has demonstrated in vitro anti-cancer properties (Johansson, S. et al., Anti-Cancer Drugs (2001) 12, 475-483) and patient profiling suggests the survival rate of cancer patients taking digitoxin is statistically enhanced (Stenkvist, B. Anti-Cancer Drugs (2001) 12, 635-636; Haux, J. et al., BMC Cancer (2001) 1, 11). Cardiac glycosides were also recently noted to inhibit the expression of four genes that are overexpressed in prostate cancer cells, including transcription factors and the apoptosis inhibitor survivin (Johnson, P. H. et al., Molecular Cancer Therapeutics (2002) 1, 1293-1304), and to provide protective effects against polyglutamine-based diseases (Piccioni, F. et al., Hum. Mol. Genet. (2004) 13, 437-446). Digitoxin also inhibits activation of the NF-κB signaling pathway in cystic fibrosis (CF) cells, suppressing hypersecretion of IL-8, a protein implicated in lung inflammation, from CF lung epithelial cells (Srivastava, M. et. al., Proc. Natl. Acad. Sci. (2004) 101, 7693-7698). Given that the attached sugars are implicated as mediators of the unique spectrum of biological properties exhibited by cardiac glycosides (Rathore, H. et al., J. Med. Chem. (1986) 29, 1945-1952), digitoxin provides an excellent model to examine the general utility of neoglycosylation to efficiently construct a glycorandomized library and to directly assess the biological impact of varying the sugars attached to a given natural product-based drug.